This most frequently happens to me when I'm building forms that rely on
classes from other .NET projects (DLLs) that I have built.

If I rebuild the other project(s) but haven't yet recompiled the
projects with the forms in it, then I get errors like this.

It seems to be a problem in the Visual Studio designer: it doesn't cope
very well with referenced dlls being out of date.

I usually close the designer windows and rebuild the project, then open
them again. Usually this resets things so that the designer is happy
again.

Sometimes the designer gets so confused that I have to close all open
windows in the project, rebuild the project, shut down Visual Studio,
start it up again, rebuild the project agian, and then I can usually
use the designer again.